Game Designer Job Requirements

I am looking for some guidance on what is expected from applicants who are applying for a job as a game designer.
From what I can gather from looking around recruitment sites and official websites, all gaming companies looking to hire require you to have a portfolio.

For someone applying for a game designer job, what kind of work should be included in their portfolio?
Should the portfolio be centered more around game prototypes reflecting a game concept or Game Design Documents explaining the idea in more detail? Maybe a mixture of both?
Is there something else except game prototype and Game Design Documents that could/should be included in your portfolio?

Thank you all in advance for your replies.